Reports: DNI650223-DNI6: Vibrational Spectroscopy of Transient Combustion Intermediates Trapped in Helium Nanodroplets

Gary E. Douberly, PhD, University of Georgia

We have made significant progress towards our research goal of implementing helium nanodroplet isolation to trap and stabilize intermediates of prototype reactions that are central to combustion chemistry.  Intermediates that result from the barrierless reactions between hydrocarbon radicals or between a hydrocarbon radical and molecular oxygen are being characterized with infrared laser spectroscopy.  Furthermore, we have successfully trapped the hydroxyl radical (OH) in helium nanodroplets and probed the outcome of the OH + O2 reaction, which sheds new light on the potential surface associated with this prototypical benchmark reaction. The vibrational frequencies, dipole moments, and vibrational transition moment angles obtained from these studies add new chemical insight into the detailed mechanisms of reactions that are relevant to low-temperature hydrocarbon oxidation and soot formation.  Furthermore, experiments of this type provide the mid-infrared spectral signatures of important combustion intermediates needed to develop laser diagnostic tools for modern chemical kinetics studies.

We reported the ro-vibrational spectrum of the nu3(e') band of the methyl radical (CH3) solvated in superfluid 4He nanodroplets.  Five allowed transitions produce population in the NK = 00, 11, 10, 22 and 20 rotational levels.  The observed transitions exhibit variable Lorentzian line shapes, consistent with state specific homogeneous broadening effects.  Population relaxation of the 00 and 11 levels is only allowed through vibrationally inelastic decay channels, and the PP1(1) and RR0(0) transitions accessing these levels have 4.12(1) and 4.66(1) GHz full-width at half-maximum linewidths, respectively.  The linewidths of the PR1(1) and RR1(1) transitions are comparatively broader (8.6(1) and 57.0(6) GHz, respectively), consistent with rotational relaxation of the 20 and 22 levels within the vibrationally excited manifold.  The nuclear spin symmetry allowed rotational relaxation channel for the excited 10 level has an energy difference similar to those associated with the 20 and 22 levels.  However, the PQ1(1) transition that accesses the 10 level is 2.3 and 15.1 times narrower than the PR1(1) and RR1(1) lines, respectively.  The relative linewidths of these transitions are rationalized in terms of the anisotropy in the He-CH3 potential energy surface, which couples the molecule rotation to the collective modes of the droplet.

Helium nanodroplet isolation and infrared laser spectroscopy was used to investigate the CH3 + O2 reaction.  Helium nanodroplets are doped with methyl radicals that are generated from an effusive pyrolysis source.  Downstream from the introduction of CH3, the droplets are doped with O2 from a gas pick-up cell.  The CH3 + O2 reaction therefore occurs between sequentially picked-up and presumably cold CH3 and O2 reactants.  The reaction is known to lead barrierlessly to the methyl peroxy radical, CH3OO.  The ~30 kcal/mol bond energy is dissipated by helium atom evaporation, and the infrared spectrum in the CH stretch region reveals a large abundance of droplets containing the cold, helium solvated CH3OO radical.  The CH3OO infrared spectrum was assigned on the basis of comparisons to high-level ab initio calculations and to the gas phase band origins and rotational constants.

The X2PI3/2 hydroxyl (OH) radical has been isolated in superfluid 4He nanodroplets and probed with infrared laser depletion spectroscopy.  From an analysis of the Stark spectrum of the Q(3/2) transition, the lambda-doublet splittings are determined to be 0.198(3) cm-1 and 0.369(2) cm-1 in the ground and first excited vibrational states, respectively.  These splittings are 3.6 and 7.2 times larger than their respective gas phase values.  A factor of 1.6 increase in the Q(1/2) lambda-doublet splitting was previously reported for the helium solvated X2PI1/2 NO radical [K. von Haeften, A. Metzelthin, S. Rudolph, V. Staemmler, and M. Havenith, Phys. Rev. Lett. 95, 215301 (2005)].  A simple model was developed that predicts the observed L-doublet splittings in helium solvated OH and NO.  The model assumes a small parity dependence of the rotor's effective moment of inertia and predicts a factor of 3.6 increase in the OH ground state (J=3/2) lambda-doubling when the B0e and B0f rotational constants differ by less than one percent.

The HOOO hydridotrioxygen radical and its deuterated analog (DOOO) have been isolated in helium nanodroplets following the in-situ association reaction between OH and O2.  The infrared spectrum in the 3500-3700 cm-1 region reveals bands that are assigned to the nu1 (OH stretch) fundamental and nu1+nu6 (OH stretch plus torsion) combination band of the trans-HOOO isomer.  The helium droplet spectrum was assigned on the basis of a detailed comparison to the infrared spectrum of HOOO produced in the gas phase [E. L. Derro, T. D. Sechler, C. Murray, and M. I. Lester, J. Chem. Phys. 128, 244313 (2008)].  Despite the characteristic low temperature and rapid cooling of helium nanodroplets, there is no evidence for the formation of a weakly bound OH-O2 van der Waals complex, which implies the absence of a kinetically significant barrier in the entrance channel of the reaction.  There is also no spectroscopic evidence for the formation of cis-HOOO, which is predicted by theory to be nearly isoenergetic to the trans isomer.  Under conditions that favor the introduction of multiple O2 molecules to the droplets, bands associated with larger H/DOOO-(O2)n clusters are observed shifted ~1-10 cm-1 to the red of the trans-H/DOOO nu1 bands.

HO3-(O2)n clusters are formed via the sequential addition of the hydroxyl radical and O2 molecules to superfluid helium nanodroplets.  Infrared laser spectroscopy in the fundamental OH stretching region reveals the presence of several bands assigned to species as large as n=4.  Detailed ab initio calculations are carried out for multiple isomers of cis- and trans-HO3-O2, corresponding to either hydrogen or oxygen bonded van der Waals complexes.  Comparisons to theory suggest that the structure of the HO3-O2 complex formed in helium droplets is a hydrogen-bonded 4A' species consisting of a trans-HO3 core.  The computed binding energy of the complex is approximately 240 cm-1.  Despite the weak interaction between trans-HO3 and O2, non-additive red shifts of the OH stretch frequency are observed upon successive solvation by O2 to form the larger clusters with n>1.